Many electronic devices and systems include integrated circuits for operating the devices and storing data during the operation of the devices. For example, electronic devices such as computers, printing devices, scanning devices, personal digital assistants, calculators, computer work stations, audio and/or video devices, communications devices such as cellular telephones, and routers for packet switched networks may include processors and memory in the form of integrated circuits. Advantages of using integrated circuits compared to other forms circuitry include space conservation and miniaturization, conserving limited battery resources, and cutting the costs of assembling the electronic devices.
Many integrated circuits include “vendor mode” circuitry that a vendor uses during the manufacturing of the integrated circuit, before the integrated circuit is sent out to a customer and incorporated into an electronic device, to test the internal operation of the integrated circuit. Vendor mode circuitry may include blocks of logic for testing whether selected features of the integrated circuit operate according to quality control specifications. Alternatively, the vendor mode circuitry may include redundant circuitry that substitutes for faulty sections of the integrated circuit in order to increase the production yield of the integrated circuits. Accessing the vendor mode circuitry may occur at a wafer level before the integrated circuit is packaged, or later at a packaged-chip level when the integrated circuit is in its final packaged form.
The vendor mode circuitry may be activated by a distinct sequence of logic values on selected pins of the integrated circuit package. Once in the vendor mode, the integrated circuit may disable the “user mode” or alternatively may operate in the vendor mode while the user mode is still active. The user mode corresponds to the command set and pin assignments available to the customer when the integrated circuit is incorporated into the electronic device and functioning. The vendor mode corresponds to another set of commands and a different assignment of pins that permit the vendor to select which test to perform on the integrated circuit, and to transfer test data between the vendor mode logic blocks and external testing equipment.
The vendor modes are intended to be available only to the vendor for testing, configuring, and/or modifying the integrated circuit internally before it is shipped to the customer. The customer is not allowed to access the vendor modes. Therefore the distinct sequence of logic values on selected pins of the integrated circuit package that activate the vendor mode are not disclosed to the customer.
During operation in the electronic device, however, the integrated circuit may accidentally receive the distinct sequence of logic values on the selected pins that cause the integrated circuit to exit the user mode and enter the vendor mode. Examples of circumstances that may give rise to the accidental generation of the vendor mode enabling logic values include noise on the selected pins, power surges, software errors that generate the distinct sequence, and user errors in providing data to the integrated circuit. When the integrated circuit accidentally enters the vendor mode, normal operation of the integrated circuit may be interrupted and the electronic device may cease to operate as intended.
In order to prevent the accidental entering into the vendor modes, the vendor may permanently disable the vendor mode circuitry before the integrated circuit is sent to the customer. For example, the vendor may blow a programmable fuse that disables the vendor mode circuitry and cannot be reset once it is blown. One example of a programmable fuse is a narrow polysilicon line on the integrated circuit that is blown by cutting with a precision laser. Another example of a programmable fuse is a narrow polysilicon line on the integrated circuit that is blown by passing a fusing current through the line.
But if an integrated circuit is returned from the customer because it is suspected of having failed in operation, the vendor may prefer to reactivate the vendor modes to test the integrated circuit and determine the cause of the failure, if any. The vendor may then determine whether the integrated circuit failed, the integrated circuit is operational and the fault lies in other components in the electronic device, or the customer accidentally enabled the vendor mode. Permanently disabled vendor mode circuitry, however, cannot be reactivated.